Caroline Wozniacki's Australian Open preparations were dealt another blow after going down 7-6(4) 1-6 6-2 to qualifier Svetlana Kuznetsova in a match that lasted almost four hours on a sweltering day at the Apia International Sydney.

The Danish former world No. 1, beaten in her first match at the Brisbane International last week, crashed out in the second round to the two-time grand slam champion who only recently returned to the tour after a six-month injury lay-off.

Temperatures in Sydney had been forecast to hit a maximum of 43C on Tuesday as Australia swelters in a heatwave that has sparked raging bush fires.

The No. 7 seed admitted afterwards that Kuznetsova's game belied her ranking of No. 85 in the world.

"For sure - she's a top player and she'll definitely get her ranking up," Wozniacki said. "Svetlana's a good player - she's a great player. She's won big tournaments in the past so she knows what it takes."

For her part, the Russian is glad to be back on court and enjoying her tennis again.

"Before the break I was losing to [Julia] Goerges, I was losing to Wozniacki, every time I was up and had chances and could never close those matches," Kuznetsova said. "Now, after the break, I just feel fresh, I feel happy, I feel balanced on the court.

"I feel I'm doing the right things when I have to do them. I'm definitely far away from where I want to be at, but I like how it looks at the moment."

Kuznetsova will face No. 2 seed Angelique Kerber in the quarter-finals after the German beat Kazakhstan's Galina Voskoboeva 6-2 7-5. Top seed Agnieszka Radwanska also progressed with a 6-4 6-3 victory against Kimiko Date Krumm, but slammed tournament organisers for allowing play to continue as on-court temperatures reached 41.4C.

"I think this is too hot to play tennis," Radwanska said. "Even for players, for ball kids, for the people sitting out there, I think it's just too hot.

"Today was one of the hottest days I played for sure."

Former French Open champion Li Na compared the conditions to playing in a sauna as she sped to a 6-1 6-0 victory against Japan's Ayumi Morita in 52 minutes.

"I was feeling, I don't know, just feeling like unbelievable. They say it was 40, but even on the tennis court even more," Li said. "I mean, even didn't finish the match, just finish first set - I was feeling my feet already burning."

Madison Keys, the American 16-year-old qualifier, produced the shock of the day by racing into a 6-0 3-1 lead against Zheng Jie, closing out the second set 6-4 to reach her first WTA quarter-final.

Italy's Roberta Vinci came back from 3-0 down in the second set and 4-2 down in the third to win another marathon match against former world No. 1 Jelena Jankovic 3-6 6-4 7-6(4) in a contest just seconds shy of four hours, while compatriot Sara Errani made far lighter work of beating Russia's Maria Kirilenko 6-1 6-1.

But Dominika Cibulkova can lay claim to the most hard-earned victory of the day. The Slovakian completed the quarter-final line-up after rallying from 5-3 down in the third set to beat Ekaterina Makarova 7-6(3) 1-6 7-6(1) in a match that lasted four hours, 46 minutes.

Another American teenager was on hand to produce the shock of the day at the Moorila Hobart International. Lauren Davis beat No. 2 Sorana Cirstea of Romania 6-1 6-3 in their rain-interrupted second-round match.

She will be joined by compatriot Sloane Stephens in the third round after the No. 8 seed saw off Simona Halep 6-4 6-0. Germany's Mona Barthel beat Chanelle Scheepers of South Africa 7-6(3) 7-5, while Serbian Bojana Jovanovski saw off Maria-Teresa Torro-Flor of Spain 6-0 7-6(7).

With top seed Hsieh Su-wei crashing out on Monday, Czech Klara Zakopalova is now the highest-ranked player still in the draw after winning her first-round match against Hungary's Timea Babos 6-4 6-3.

And a bad day for the top seeds continued when Elena Vesnina of Russia knocked out No. 4 seed Yaroslava Shvedova 4-6 6-2 6-1 in the second round.